Basic Singing Query

I have some basic doubts about singing.
In most of the music schools in initial stage of learning vocal music they teach hindustani vocal music where students asked to match their voice with that of harmonium / tanpura.

Why are students asked to match their voice with that of harmonium / tanpura?

How does matching the student singers voice with harmonium sound helps?
Is not only singing sa re ga ma in any pitch sufficient?